Abstract

     Anchorage of shear reinforcement, such as links or stirrups, can be performed by providing hooks, bends or heads at their ends, by welding transverse reinforcement or by closing open stirrups with pins. Hooks and bends, also used to enhance the anchorage of flexural reinforcement at the end regions of beams and slabs, have often been preferred because of their simple and cost-effective production. Such anchorages present nevertheless several peculiarities that shall be accounted for. They are relatively sensitive to their detailing (mandrel diameter, length of the tail segment between bar end and bent region) as well as to the cracking state of the surrounding concrete. Also, brittle failures can occur due to spalling of the concrete cover in case of bars near to a free surface.

Introduction

     Since the beginning of reinforced concrete construction, mechanical anchorage of the reinforcement in tension by means of bends and hooks has been extensively used in a large number of applications, such as beams, walls and slabs (Fig. 1a-f). Arrangement of hooks was first required for the anchorage of plain bars due to their poor bond performance. Such solution was observed to be efficient and was also adopted for the anchorage of various types of shear and punching reinforcement.

     The first research works on hooks were performed for the anchorage of flexural reinforcement at the extremity of beams. On the basis of beam tests, Hyatt in 1877 [1] observed that flat reinforcing bars bent at 90° at their ends gave higher performances. As a result of this work, 90° bends became popular in the following years in the USA [2]. In Europe, in 1908 Mörsh [3] encouraged the use of hooks to enhance the performance of anchorages. He also proposed to use higher bend angles than 90°, in contrast to the practice in the USA. The reason for this was that 90°-angle bends caused failure issues in the case of too small concrete cover (as demonstrated by the tests of Wayss and Freytag [4], [5]). It was Considère in 1907 [6] who formally proposed a 180° hook bent with a mandrel diameter equal to 4 times the bar diameter to avoid failures due to concrete crushing. In addition to the use of hooks at beam ends, the hook anchorage of shear reinforcement (Fig. 1a-e) was also used in the early developments of reinforced concrete.

Conclusions

     This paper presents the results of an experimental programme and a mechanical model allowing to better understand the behaviour and to predict the resistance of bends and hooks in tension. The case of the anchorage of shear reinforcement near to a free surface is investigated. In addition, the influence of bending cracks on the performance of shear reinforcement is also investigated. The main conclusions are listed below:

1. Three failure modes potentially govern the strength of a bend anchorage: (i) spalling of the concrete cover, (ii) pull-out (bond) failure and (iii) reinforcement yielding. The governing failure mode depends upon the size of the concrete cover, the transverse crack opening and the detailing of the region.

2. The mechanical response of bend anchorages is complex, with an interaction between normal, bond stresses and longitudinal steel stresses due to axial forces and bending. Such interaction is confirmed by means of an experimental programme performed with detailed fibre-optic measurements. Also, the experimental results presented in this paper show a significant influence of the state of cracking, consistently with previous researches on bond.

3. Spalling of the concrete cover is originated by a combination of the tensile stresses associated to bond and the uplift forces in the tail region of bends (lever effect). Such uplift forces allow, by equilibrium conditions, to develop an additional tensile force in the reinforcement at the end of the bend (enhancement of the anchorage capacity in case of pull-out failures).

The bond stress of a reinforcing bar in a cementitious matrix varies along the bar length and is difficult to quantify. Thus, design code provisions refer to the concept of bar development length and rely on statistical analysis of rebar-pull-out test results. In the present study, a novel data-driven predictive model based on Polynomial Chaos Expansions (PCE) was developed to predict the reinforcing bar development length using 534 experimental results of simple pull-out tests on short unit bar lengths. The predictive capability of PCE was compared to that of other data-driven models, namely the Response Surface Method (RSM) and Artificial Neural Networks (ANN). Moreover, predictions of the PCE, RSM and ANN were further compared with calculations of three commonly used design code formulas (i.e., ACI 318-14, ACI 408R-03, and Eurocode 2) and predictions of two existing empirical models (i.e. Model Code 2010 and Hwang et al. model). A parametric study was conducted to explore the sensitivity of the proposed model to influential input parameters. It was found that the Polynomial Chaos Expansions model offers a powerful predictive tool for reinforcing bar bond strength. The model was able to capture trends that differ from that of existing models that assume unrealistic uniform bond stress along the rebar. This flexible and data intensive model for predicting rebar bond stress and full embedment length could offer an intelligent platform for accommodating new bar materials, new test data, and calibrating existing design provisions to keep design codes relevant.

Introduction

Reinforced concrete technology has been undergoing transformative changes in recent decades. For instance, various types of reinforcing steel rebar are being used, including carbon steel with various deformed shapes, galvanized, epoxy-coated, stainless, and high-strength steel rebar [1,2]. Moreover, new composite material bars with different engineering properties and surface texture have emerged as contenders for concrete reinforcement, including carbon, aramid, glass and basalt rebar. Likewise, the cementitious matrices in which various rebar can be embedded have become diverse. Indeed, different types of concrete have become mainstream with inherently different bond to and compatibility with reinforcing rebar. These include conventional concrete, mass concrete, cellular concrete, fiber-reinforced concrete, highstrength concrete, reactive powder concrete, ultrahigh-performance concrete, polymer-modified concrete, lightweight concrete, pervious concrete, shotcrete, rubberized concrete, roller-compacted concrete, self-consolidating and anti-washout concrete, concrete using calcium aluminate or calcium sulfo-aluminate cements, just to name the most common types. Moreover, sustainability considerations have brought about eco-efficient concrete types such as geo-polymer concrete, concrete using alkali-activated binders, and other alternative cementitious matrices.

The utilization of marine resources in reinforced concrete is severely limited, due to the risk of steel corrosion caused by the introduced chloride. In order to reduce this risk, nano-Al2O3 (NA) was added into cement-based materials system by improving the chloride immobilization. The immobilized chloride ratio (ICR) of the system was evaluated, and the results indicated that the addition of NA could significantly enhance ICR. Based on the results from TG, XRD, 27Al NMR, MIP, 29Si NMR, and EDS, the mechanism behind was discussed from three aspects of chemical binding, migration resistance, and physical adsorption. The reasons for the increased ICR were as follows: a) chemical binding was promoted by increasing the amount of chloroaluminates; b) migration resistance was improved by refining the pore structure; c) physical adsorption, depending on the amount and the Ca/Si ratio of C-S-H gel, was reduced negligibly. These findings can provide guidance for the theoretical study of chloride immobilization and the application of marine resources.

Introduction

The utilization of marine resources, such as sea sand and coral sand, has great potential in marine concrete engineering, especially in reef construction in the distant sea. However, the direct use of these resources has been banned in reinforced concrete, due to the potential risk of steel bar corrosion induced by the diffused chloride ions. In order to resist this risk, corrosion inhibitors are usually used in concrete to produce a protective film on the surface of rebar [1–3]. However, due to the existence of large amounts of internal free chloride ions, the risk of pitting corrosion is not completely eliminated by inhibitors. In this case, the chloride immobilization is considered to be another promising way [4,5]. The mechanism of chloride immobilization can be divided into three aspects: chemical binding, physical adsorption, and migration resistance [6,7]. Chemical binding is related to the chloride ions bound in Friedel’s salt (FS) or Kuzel’s salt (KS).

Plant fibers (especially, wheat straw) are available surplus to requirements in sub-tropical regions. Many researchers have studied these fibers for non-structural applications. However, for civil engineering structural applications, in depth behavior of wheat straw reinforced concrete (WSRC) with steel rebars is not known. For this purpose, WSRC needs to be explored in detail for load bearing structures. This paper presents the contribution of plant fibers (i.e. wheat straw) in improving the behavior and capacity of reinforced concrete for structural applications. Reinforced concrete beam-lets with varying flexure and shear rebars, without and with inclusion of wheat straw, are experimentally investigated for studying the altered behavior due to fibers. In addition, to start with the practical implications, concrete pavements are considered. The study is concluded with an increase in flexural strength (up to 7.5%), energy absorption (up to 30.4%), and toughness indices (up to 11.1%) along with better crack arresting mechanism by incorporation of wheat straw in reinforced concrete. Also, concrete pavement containing wheat straw has comparable design with likely more durable and sustainable structure.
2018 Elsevier Ltd. All rights reserved.

Introduction

Concrete is a most widely used construction material all over the world. It is basically very strong in compression however it is brittle in nature. Brittleness of concrete results in low strain capacity in tension and thus ultimately have low toughness. Many researchers have been working on increasing the toughness of concrete with the addition of dispersed fibres. So, it has been long recognized as a solution for enhancing the energy absorption capacity, toughness and crack resistance or crack arresting [1–5]. This technique had been used since biblical times for strengthening the brittle matrices. The development of cracks and increase in width of already developed cracks can be prevented using dispersed fibres as crack arresters [6,7]. Biryukovichs used glass as dispersed fibre for the reinforcement of cement paste and mortar in early 1900 [8]. The application of dispersed fibres in the concrete as a building material for different purposes resulted in improved properties [9]. However, the incorporation of fibres in steel reinforced concrete has also been studied by various researchers [7,10–14]. Shear behavior of fibre reinforced concrete (FRC) beams was studied by [11]. Polypropylene fibres with 0.5% and steel fibres with 0.5%, 1%, and 2% content, by volume of wet concrete, were used. The compressive strength and tensile strength were increased up to 54.8 MPa and 4.3 MPa, respectively. High strength fibre reinforced concrete beams with steel rebars were also studied by [13]. The steel ratio used were 0.0017, 0.0064, 0.0075, 0.012, 0.015, and 0.022 for tensile reinforcement. And for compression reinforcement, 0.0045 and 0.0047 steel ratios were used. The percentages content for the steel fibres used were 0.5%, 1%, and 2%, by volume of wet concrete. The study resulted in significant increase of flexural strength. The above-mentioned studies concluded that the fibre reinforced concrete with steel bars showed improved results in crack and deflection resistivity, toughness, and energy absorption.

Strength, stiffness, efficiency and economy of Reinforced Concrete (RC) make it most widely used construction material in the whole world. Utility of RC, being the most widely used material is derived from the combination of two materials; concrete matrix which is strong and durable in compression and steel which is strong and ductile in tension. In order to maintain composite action between two materials, transfer of stresses between them through bond is essential (Dominguez and Ibrahimbegovic, 2006; Wang and Liu, 2003). In predicting the behaviour of RC structures close to reality, consideration of bond-slip phenomenon is must (Jendele and Cervenka, 2006; Mazzarolo et al., 2012). Currently, various numerical models are available to represent the steel-concrete bond behaviour. Among them, some models consider the introduction of zero thickness elements at steel-concrete interface (Casanova et al., 2012). In the past, a spring element to relate the steel and concrete nodes was also proposed by Ngo and Scordelis (1967). Many research studies have been carried out to simulate the behaviour of reinforced concrete structural elements taking into account behaviour of bond between concrete and steel rebar such as model based on plasticity theory (Cox and Guo, 2000) or the damage theory (Ragueneau et al., 2006; Willam et al., 2004) but few studies were focused on the role of interface between concrete and rebar in the cracking process of the concrete. In recent years, different finite elements have been proposed to model the bond effects in RC structures (Radtke et al., 2010). The models which are based on interface elements (joint elements) are more difficult to use because they need an oriented mesh of the interface with a volume description of the bar which costs more in terms of computation resources and time. Recently a new bond slip model for reinforced concrete structures has been proposed by Mang et al. (2015). This type of model needs an intensive modifications of FE code (new element type not simply constitutive law), as it is not allowed in all commercial codes. Considering this kind of restrictions, an approach based on plastic interface presents a practical interest. In this contribution, an approach to model the steel-concrete interface by introducing massive bond element with elastoplastic isotropic behaviour is proposed. Experimental pull-out test is always considered to be the main tool which provides necessary information required for modelling of bond between steel and concrete (Pyo and Lee, 2010). Stress–strain behaviour curve required for elasto-plastic material model was obtained through performing pull-out tests on concrete samples as per RILEM Standards (RILEM 7-II-128, 1994). After description of the proposed bond element, its validation through uni-axial pure tensile loading test on RC prism and flexure tests on RC beam is discussed in this paper. The finite element code CASTEM (Cast3M, 2000) is used to perform finite element analysis. The detail description of the FEM code CASTEM and as well as detailed theoretical formulation of used damage model of concrete is not the purpose of this paper. The latter has already been published (Sellier et al., 2012a; Sellier et al., 2012b).